In recent years, promotion of energy saving is strongly demanded from a viewpoint of preventing global warming. With regard to electrical home appliances, too, energy saving is an urgent task to be tackled. Particularly, in the field of refrigerators and freezers where efficient use of heat is important, high-performance heat insulators are in high demand.
Insulators made of such fiber as glass wool or such polyfoam as polyurethane foam are in general use. To increase the insulation performance of such insulators, it is necessary to increase their thickness. Therefore, they cannot be used in cases in which space where insulators can be filled is strictly limited or in cases in which highly effective use of space is imperative.
Under the circumstances, vacuum insulation panels have been introduced for use as high-performance heat insulators. A vacuum insulation panel is a heat insulator produced by putting a core, which plays a role of spacer, in a gas-barrier-performing outer barrier bag, depressurizing the interior of the outer barrier bag and sealing the outer barrier bag. Such a vacuum insulation panel, for example, like the one disclosed in Japanese Patent Laid-Open No. H9(1997)-138058 has a core made of such fiber as glass wool cured and shaped using an organic binder.
A core made of such fiber as glass wool cured using a binder, however, may scratch or tear, with burrs formed on it, an outer barrier bag when it is put in the outer barrier bag. Hence, a vacuum insulation panel having a core produced using no binder has been proposed.
Such a vacuum insulation panel to be produced using no binder has an inner film bag in which such fiber as glass wool is accommodated. The inner film bag is compressed, its interior is depressurized, and its opening is seal-welded. An example of such a vacuum insulation panel is disclosed in Japanese Patent Laid-Open No. H4(1992)-337195.
When vacuum insulation panels are used in a refrigerator, in a space filling foam insulation between an outer box and an inner box of the refrigerator, the vacuum insulation panels may be placed toward the outer box, toward the inner box, or in intermediate positions between the outer box and the inner box. In practice, they are placed toward the outer box. To be more concrete, they are, in many cases, attached to an internal surface of the outer box using two-sided adhesive tape or hot-melt adhesive.
There are reasons why the vacuum insulation panels are placed toward the inner box in fewer cases. There is an advantage to placing vacuum insulation panels toward the inner box. That is, the area to which vacuum insulation panels are to be applied is smaller when the vacuum insulation panels are placed toward the inner box than when they are placed toward the outer box. There are however problems associated with placing the vacuum insulation panels toward the inner box. The inner box, for example, is more prone to get deformed than the outer box, and the outer surface of the inner box is more irregular than the inner surface of the outer box. Therefore, it is difficult to firmly fix vacuum insulation panels to the outer surface of the inner box. In addition, when foam insulation is filled between the inner box and the outer box, cavities tend to be formed between the vacuum insulation panels and the inner box. Such cavities if formed may cause the inner box to be deformed and the insulation performance of the refrigerator to decline.
Patent literature 1: Japanese Patent Laid-Open No. H9(1997)-138058.
Patent literature 2: Japanese Patent Laid-Open No. H4(1992)-337195
In a case where a core to be put in a vacuum insulation panel is produced using a binder, an organic or inorganic fiber laminate is cured into a board-like shape using an organic or inorganic binder. The cured fiber laminate is then cut into a prescribed size, for example, using a press. The core produced in this way can have a stable shape. Also, the core having been cured is easy to handle. There are however problems posed by the core produced in the above-described way. For example, when such a core is put in an outer barrier bag or when the interior of an outer barrier bag in which such a core is put is depressurized, burrs formed in end portions of the core when it was cut using a press may scratch or tear the outer barrier bag. Furthermore, with the core being a hard board, when the interior of the outer barrier bag in which the core is put is depressurized, the core, without changing its shape, scratches or tears the outer barrier bag with burrs formed in its end portions. Also at the time of the depressurization, convection spaces looking like erected tents are formed between the outer barrier bag and end faces of the core. If the core gets deformed in the process of production, it results in a deformed, for example, warped vacuum insulation panel. Such a vacuum insulation panel may cause trouble when it is attached to the outer box of a refrigerator.
The convection spaces are formed more easily when the board-like core is thicker or when the outer barrier bag is less flexible.
There is another problem with a core produced using a binder. Suppose, for example, vacuum insulation panels, their cores in particular, are to be removed for recycling from a refrigerator which was in use for 10 years. If a fiber laminate making up the core of a vacuum insulation panel used in the refrigerator is impregnated with a binder, it is powderized when the core is taken out and broken down. At such a time, the fiber laminate is powderized into irregularly sized particles which cannot be conveniently recycled. It is difficult to remove the fiber laminate and the binder separately and re-shape the fiber laminate to produce a new core.
The vacuum insulation panel disclosed in Japanese Patent Laid-Open No. H4(1992)-337195 has a core produced by compressing a fiber laminate making use of an inner film bag without using any binder and depressurizing and shaping the inner film bag in which the fiber laminate is put. The core thus produced is put in an outer barrier bag thereby producing a vacuum insulation panel.
To be more concrete, the vacuum insulation panel disclosed in Japanese Patent Laid-Open No. H4(1992)-337195 is produced as follows: an inner member (core) is produced by putting an inorganic fiber mat in an inner film bag made of plastic film, compressing and depressurizing the interior of the inner film bag and seal-welding the inner film bag opening; the inner member (core) is put in an accommodating member (outer barrier bag); the inner film bag is broken; the interior of the accommodating member is depressurized; and the accommodating member (outer barrier bag) is seal-welded. This method allows use of a glass wool mat as a core material to be put in the vacuum insulation panel, so that the heat insulation panel is described as having remarkably improved heat insulation performance compared with vacuum insulation panels using conventional core material such as foam perlite or inorganic powder.
The core used in the vacuum insulation panel disclosed in Japanese Patent Laid-Open No. H4(1992)-337195 is not like a board. It is therefore free of burrs which are formed when a board-like core material is cut and which may damage the outer barrier bag in which it is put. It does not get warped, and convection spaces looking like erected tents are not easily formed between end faces of the core and the outer barrier bag. In the Japanese Patent Laid-Open No. 4-337195, however, no measure for making convection spaces, which may be formed, smaller is proposed, and no consideration is given to convection spaces which may be formed where an edge of the outer barrier bag is processed.
When an outer barrier bag is made of aluminum foil, it is superior in terms of gas-barrier characteristic. A problem with such an outer barrier bag is that, with the heat conductivity of aluminum itself being high, heat conduction (heat bridging) via the outer barrier bag makes it difficult to achieve adequate heat insulation performance.
The present invention provides a vacuum insulation panel, which is free of convection spaces as described above, which has superior heat insulation performance, which makes, in the production process, edge folding easy, and which can be produced with high productivity, and also a refrigerator incorporating the same.
According to the present invention, a refrigerator incorporating vacuum insulation panels with high recyclability can be provided.
According to the present invention, a vacuum insulation panel free of binder can be provided by using a binderless core which does not cause the vacuum insulation panel to warp and which is advantageous in securing plane flatness accuracy.